We have a 2006 Mainship with 3 HVAC units having a problem with two of the units when on blowing out a white powder like substance. This substance is real fine have cleaned all the filters and still doing it any ideas would appreciate

Possibly aluminum oxide from the coil, or ducting; check the pH of the condensate, it should be neutral (7).

For curiosity, are all 3 units the same? Where are the intakes of each and have you recently started doing something different around the intakes of 2...something as innocent as burning incense; anything that would change the air quality or moisture?

It is amazing how many folks ignore their AC units season after season and still expect them to function as normal.
The best thing you can do for them each season is clean the coil.
BUT if you touch them with a brush or anything like it you will ruin the coil.
Just spray, let sit for a few minutes and rinse with a lazy stray from a low pressure garden hose..
This is the stuff to use and at an amazing price. One can per AC unit does the job with a couple application and rinses.
Be Sure And Wear Safety Glasses.